Biogas, which is a mixture of several gases, is typically produced by the breakdown of organic matter in low oxygen conditions. In particular, it may be produced by the anaerobic digestion or fermentation of organic matter (e.g., manure, sewage sludge, municipal solid waste, biodegradable waste, biodegradable feedstock, etc.).
Biogas collected at its source (e.g., a landfill or anaerobic digester) may be referred to as raw biogas. The composition of raw biogas, which may vary with the type of organic matter from which it is derived, is predominately methane (CH4) and carbon dioxide (CO2), with small and/or negligible amounts of nitrogen (N2), hydrogen sulfide (H2S), water (H2O), ammonia (NH3), hydrogen (H2), carbon monoxide (CO), oxygen (O2), volatile organic compounds (VOCs), and/or siloxanes. For example, without being limiting, the composition of raw biogas may include about 60% CH4 (e.g., between about 35% and about 75%), about 35% CO2 (e.g., between about 15% and about 65%), about 0-20% N2, and about 0-5% O2.
While raw biogas may have sufficient energy content to be combusted (e.g., in an engine, turbine, or boiler) without removing inert components such as CO2 and/or N2, another option is to upgrade the raw biogas for injection into a natural gas distribution system. Biogas upgrading refers to a process that increases the calorific value of biogas by removing at least CO2 and/or N2, and typically some other contaminants, thereby increasing the relative amount of CH4. When raw biogas is upgraded to the extent that it meets applicable specifications of the natural gas distribution system (e.g., pipeline standards) and/or is suitable for use in the transportation sector, it is referred to as “renewable natural gas.” Renewable natural gas (RNG) is substantially interchangeable with natural gas and thus can be used as a substitute for fossil natural gas, can be injected into the natural gas distribution system, and/or can be used as a transportation fuel where it can qualify for fuel credits.
In general, when raw biogas is upgraded to RNG, the biogas upgrading is associated with relatively high capital investment costs, and as a result, is not generally economically feasible for biogas producers having a low volume production (e.g., small farms, small landfills, or small wastewater treatment facilities).